Generally, a lightweight alloy using a lightweight material such as aluminum or magnesium is widely used in various industrial fields including in transporting apparatus such as motor vehicle or airplanes, and in the engineering and construction industries. In order to manufacture such apparatus and structures, joining such lightweight alloys together through welding occurs frequently.
Therefore, a technology for welding such lightweight alloys is widely studied, and recently friction stir welding (FSW) has been introduced. Regarding the FSW technology, an international publication document (international publication number: WO93/10935; applicant: THE WELDING INSTITUTE) can be referred to.
Before the friction stir welding technology was introduced, friction welding was used for several tens of years. Friction welding joins workpieces together by creating a plasticized zone around a contacting surface of the workpieces by generating sufficient frictional heat in workpieces that will be joined together, and then applying an external force. However, because such friction welding heats the contacting surface of the workpieces by rotating the workpieces, there is a restriction in that at least one of the workpieces must be axis-symmetrical. As an example, friction welding cannot be used to weld edge surfaces of plates. Therefore, friction welding is only applicable in a restricted range.
The friction stir welding technology generates friction between workpieces and a welding tool, rather than friction between the workpieces, and uses frictional heat generated by the friction.
FIG. 1 is a drawing for explaining the friction stir welding technology.
As shown in FIG. 1, after workpieces 130 that are to be joined are firmly fixed to each other, a portion (generally called a pin) 120 of a rotating tool 100 is inserted between the two workpieces 130 at a contacting surface (i.e., a welding line) 140. Therefore, if the tool 100 is rotated while the pin 120 is inserted in the contacting surface 140, a joining portion around the contacting surfaces of the workpieces 130 is heated by frictional heat between the tool 100 and the workpieces 130, so a plasticized zone are created in the workpieces 130. At this time, if the tool 100 is moved along the welding line 140 by a mechanical force, the heated joining portion is pressurized to move from a front portion of the tool 100 and to a rear portion thereof, and a solidified joining portion is formed through a combination of the frictional heat and the mechanical processing. Through such processes, the workpieces 130 are welded together.
Recently, efforts to further develop friction stir welding by utilizing and improving the same have been made. As examples, an international publication document (international publication number: WO95/26254, applicant: NORSK HYDRO A.S) and a United States patent document (U.S. Pat. No. 5,829,664, Assignee: Aluminum Company of America) are instances of such efforts.
In WO95/26254, attempts to improve an applicable range and characteristics of friction stir welding by inclining a welding tool with respect to a welding line, by improving a shape of a bottom surface of a welding tool, or by improving a shape of a pin, were made.
In U.S. Pat. No. 5,829,664, an attempt to improve productivity and quality of friction stir welding by more effectively heating a joining portion through electrical heat generation in workpieces through application of an electric current from an electric power supply as well as frictional heat was made.
However, in the friction stir welding technology thus far, there is much room for improving heating of a joining portion of workpieces.
As an example, according to the U.S. patent document that attempts to obtain additional heat generation by applying an external electric current, the external electric current must be very high in order to sufficiently heat the workpieces, so there is a drawback in that a power supply having large electric capacity is needed. In addition, so as to apply an electric current to the workpieces from the electric power supply, an additional device (in the above-stated U.S. patent document, a conducting table is used) is needed. Furthermore, the electric current does not regionally flow through a joining portion of the workpieces but also dissipates through grounded locations, so that the technical effect is not particularly substantial and various defects are generated.